1. Field of the Invention
The present invention relates to a method for multiplexed reading or writing of large sized files such as those of the video data, and more particularly, to a method for multiplexed reading or writing of data capable of improving the multiplexed operation performance in the video-on-demand system at a time of executing a plurality of reading requests or writing requests simultaneously with respect to a memory device storing a multiplicity of video data accompanied by audio data.
2. Description of the Background Art
In the conventional video-on-demand system in which a center device and a number of terminals are connected, when a terminal requests a reception of a desired video program to the center device, the center device reads out data of the requested video program from a memory device and transmits the read out video data to the requesting terminal.
In such a case, there is a possibility for the other request from the other terminal to arrive at the center device while reading out the data of the requested video program from the memory device in response to the request from one terminal, and in order to deal with such a situation, it is necessary to provide a multiplexed reading/writing operation with respect to the memory device. To this end, there has been a proposition of the multiplexed reading device as disclosed in Japanese Patent Application Laid Open No. 4-269087 (1992) which is capable of executing the reading operation with respect to the memory device (magnetic disk device) at a higher rate than the bit rate of the video program in time sharing basis.
Namely, in this conventional multiplexed reading device, the compression encoded video data are stored on disks, and the utilization time-slots TS.sub.1 to TS.sub.n of the disk device are assigned sequentially in this same order at every one of the first and subsequent operation periods. On the disks to be reproduced by the disk device, each video data is recorded by being divided into a number of pieces, such that when these divided pieces are sequentially read out by making accesses at the first time-slot TS.sub.1 of the first period, the first time-slot TS.sub.1 of the second period, and so on up to the first time-slot TS.sub.1 of the n-th period, and then joined together, one continuous video data can be obtained. Similarly, the access and reading of the video data are carried out such that when the data read out at the same i-th time-slots TS.sub.i of the different periods are joined in a correct order, one continuous video data can be obtained. The data reproduced from the disks in this manner are then written into a buffer memory for the purpose of expanding the compression encoded video data by an expansion device to recover the normal video data, and the recovered normal video data are transmitted to the terminal to display the visual images according to the normal video data at a display monitor of the terminal.
Now, in order to increase the number of multiplexed reading operations at the center device as a whole, it is possible to consider a system configuration in which a plurality of multiplexed reading devices as described above are provided in parallel.
FIG. 1 shows an exemplary configuration of such a system in which a plurality of multiplexed reading devices are provided in parallel. This configuration of FIG. 1 includes a number of terminals T.sub.1 to T.sub.40+a connected to a network 2, where each terminal includes a communication control unit, a decoder, an AV device, and an input device (not shown) such that the request of the user specified at the input device are transmitted to the center device by the communication control unit, while the video data received from the center device are decoded (expanded) by the decoder and displayed by the AV device.
The network 2 is formed by the ISDN (Integrated Services Digital Network) for example, and equipped with a number of input/output channels C.sub.1 to C.sub.40, where each input/output channel is in a form of a board mounting a buffer memory and a communication processing device (not shown).
The configuration of FIG. 1 further includes four memory devices M.sub.01 to M.sub.31 such as the magnetic disk devices, which are connected with the input/output channels C.sub.1 to C.sub.40 through memory control units MC.sub.01 to MC.sub.31 and switching units SW.sub.0 to SW.sub.3. Here, four memory control units MC.sub.01 to MC.sub.31 are provided within the center device, where each memory control unit is in a form of a magnetic disk control board. In a case of the SCSI (Small Computer System Interface), one memory control unit can be connected with up to seven memory devices in series to expand the memory capacity.
The switching units SW.sub.0 to SW.sub.3 are formed by the VME (Versa Module Europe) bus which constitutes the transmission paths between the memory control units MC.sub.01 to MC.sub.31 and the input/output channels C.sub.1 to C.sub.40. In this example, a number of time-slots in each period is set to 10, so that each switching unit connects ten of the input/output channels to one memory control unit and each memory control unit has a capacity to handle ten multiplexed reading/writing operations within each period.
The configuration of FIG. 1 also includes a multiplexed reading/writing control unit 31 formed by a communication control unit and a micro-processor (not shown), which admits the requests from the terminals T.sub.1 to T.sub.40+a, and issues commands for the multiplexed reading/writing operations to the memory control units MC.sub.01 to MC.sub.31 and the switching units SW.sub.0 to SW.sub.3.
In this configuration of FIG. 1, when a user at one of the terminals T.sub.1 to T.sub.40+a requests a desired video program, the request is transmitted to the multiplexed reading/writing control unit 31 within the center device through the network 2. In response, the multiplexed reading/writing control unit 31 searches out an appropriate one of the memory devices M.sub.01 to M.sub.31 which stores the data of the requested video program, and commands one of the memory control units MC.sub.01 to MC.sub.31 connected with the searched out appropriate memory device to execute the reading of the data of the requested video program.
Here, the multiplexed reading/writing control unit 31 allocates the input/output channels C.sub.1 to C.sub.10 as the output destinations of up to ten video program requests with respect to the memory control unit MC.sub.01 in an order of arrivals, and returns a center busy signal to all the terminals requesting the requests in excess of ten. The data of ten video programs are read out from the memory device M.sub.01 in units of segments at ten time-slots of each period, respectively, through the memory control unit MC.sub.01 under the control of the multiplexed reading/writing control unit 31, and transmitted to the respective allocated input/output channels C.sub.1 to C.sub.10 through the switching unit SW.sub.0. The other memory control units MC.sub.11, MC.sub.21, and MC.sub.31 are similarly connected with the other input/output channels C.sub.11 to C.sub.20, C.sub.21 to C.sub.30, and C.sub.31 to C.sub.40 through the other switching units SW.sub.1, SW.sub.2, and SW.sub.3, respectively.
Thus, in this case, the input/output channels C.sub.1 to C.sub.40 and the memory control units MC.sub.01 to MC.sub.31 are connected in correspondence relationship as summarized in the table shown in FIG. 2. Namely, the memory control unit MC.sub.01 is sequentially connected with the input/output channel C.sub.1 at the first time-slot TS.sub.1 of each period, the input/output channel C.sub.2 at the second time-slot TS.sub.2 of each period, and so on. The connection and reading operations by the input/output channels C.sub.1 to C.sub.40 and the memory control unit MC.sub.01 to MC.sub.31 according to this correspondence relationship of FIG. 2 is repeated in each period. Each of the input/output channels C.sub.1 to C.sub.40 transmits the segment data of the requested video programs received from the connected memory control units MC.sub.01 to MC.sub.31 to the respective requesting terminals through the network 2. Then, at each terminal, the sequentially transmitted series of segment data for the requested video program are decoded and displayed.
In this conventional multiplexed reading device of FIG. 1, four of ten-fold multiplexed sub-systems are provided in parallel to improve the multiplexed reading capacity of the center device to up to 10.times.4=40, but this type of the conventional multiplexed reading device has a problem in that the requests in excess of ten with respect to each memory device cannot be admitted even when a large number of requests from the terminals are concentrated onto a particular video program stored in a particular memory device, no matter how many sub-systems are provided in parallel. In other words, in this type of the conventional multiplexed reading device, when m sets of n-fold multiplexed sub-systems are provided in parallel, regardless of how large the number m of the sub-systems provided in parallel may be, the maximum number of requests that can be admitted simultaneously with respect to each memory device is limited to the number n of the multiplexed reading operations supported by each sub-system, so that the service of a particular video program can be provided with respect to only up to n terminals simultaneously even when more than n terminals requested this particular video program.